Most current switches and trunks are four-wire, and the loops are
two-wire. If the impedance match at the four-wire to two-wire conversion point
is not perfect, some of the transmitted energy reflects back to the receiver
path. The transmission delay path becomes critical. When it is great enough,
the users hear an echo of their voice. In other words, reflections that occur
at the far end of the connection are not eliminated if the round-trip transit
delay is greater than the echo-canceller memory. With the introduction of Voice
over IP (VoIP), the data network adds to the transmission delay. In most
configurations, the network deploys echo chancellors at all Public Switched
Telephone Network (PSTN) interfaces. Echo Control Devices (ECDs) controlled by
the logic described in International Telecommunication Union Telecommunication
Standardization Sector (ITU-T) Recommendation Q.115 are designed to eliminate
the echo of a voice or audio signal.

This document describes the basic items for the Cisco PGW 2200 in the
way Media Gateway Control Protocol (MGCP) packets are sent to the gateway. The
technique used in these ECDs to eliminate the echo is beyond the scope of this
document.

The information in this document is based on the following software and
hardware versions:

Cisco PGW 2200 Software Release 9.x and later

The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.

The MGCP organizes the communication between a PGW 2200 (Call Agent
[CA]) and a media gateway (AS5xxx) (such as a VoIP gateway or a Network Access
Server [NAS]). This document explains how the two options (L:e on, L:e off) in
the MGCP message work.

Based on the image in Call Flow,
below, when the PGW 2200 (or CA1 in the image) receives an Initial Address
Message (IAM), the PGW 2200 (or CA1) immediately sends a Create Connection
(CRCX) request to the trunking gateway to connect to the incoming trunk. The
CRCX is built up as shown in the following example:

The PGW 2200 only sets echo control to
"on" on the originating side of MGCP calls
through the CRCX connection and the L: e:on.
However, it is necessary for some calls to turn off these operations. The echo
cancellation (ECAN) parameter e: can have two
values: on (when the ECAN is requested) and
off (when it is turned off).

The terminating side has L: e:off (echo
off) in the MGCP CRCX. Because the switch thinks that it is not the latest
switch (or the end switch) in the network, it turns the echo off, since echo
happens at the end points nearest to the phone.

In determining the echo suppressor field, the most important part of an
IAM Signaling System 7 (SS7) message is Echo Suppressor
Ind. The following is an example of an IAM SS7 message:

The ECD Bit = 0 or 1 in
Scenario 1, below, is the ECD bit in the
Nature Of Connection parameter and in the
Backward Call Indicators parameter of the
Address Complete Message (ACM) SS7 message.

In the IAM or ACM , the ECD bit is linked to:

ECD=0—There is no ECAN on this leg, so
it needs to be enabled.

ECD=1—ECAN is already on this leg, so
it does not need to be enabled.

For a generic network architecture on the PGW 2200 switched solution,
you can change the EchoCanRequired parameter to
match the requirement of the command below.

The command to enable ECAN for a trunk group is the following:
prov-add:trnkgrpprop:name=trunk_name,EchoCanRequired="1".

Note: In a SS7 terminated call, after in
answer state. If the CPG with BCI is received
from SS7 side and "EchoCanRequired=1" set on trunkgroup or sigpath, a MDCX is
sent to GW to enable/disable the echo control. This is the behavior:

EchoCanRequired=1
1. if echo device in BCI =0, MDCX will sent down with e: on
2. if echo device in BCI =1, MDCX will sent down with e: off
EchoCanRequired=0
no MDCX will be sent down for echo control.

Note: This is a general call flow model of an MGCP call-setup situation.
There is an Extended-ISDN User Part (E-ISUP) link between CA1 (PGW) and CA2
(PGW).

For an incoming IAM message with the ECD bit set to 0, the PGWA sends a
CRCX [e:on] to Trunking Gateway A (TGWA). By
E-ISUP, which has ECD equal to 1, the details are forwarded to PGWB . The
second row of Scenario 1, below, shows how
the ACM forwards the details to PGWA.

In this scenario, the public SS7 switches at both ends are equipped
with ECD. If the ACM message Echo control device
indicator is set (Bit=1), the
PGW does not send the modify with "echo off". PGWA should not send a Modify
Connection (MDCX) (e:off) on ACM to the near-end
gateway; otherwise, the far-end user hears an echo.